When the 'disabled-by-default-v8.gc' category is enabled, emit an instant event
with heap statistics after every GC. The data that's emitted is the same as what
the V8 API gives you with `GetHeapStatistics()` and `GetHeapSpaceStatistics()`.

We generate JSON with the following format:

```
{
 "isolate": "0x55dd5cf03b50",
 "id": 1,
 "time_ms": 42.619,

 "total_heap_size": 3981312,
 "total_heap_size_executable": 573440,
 "total_physical_size": 2820440,
 "total_available_size": 2195254440,

 "used_heap_size": 1799616,
 "heap_size_limit": 2197815296,
 "malloced_memory": 251024,
 "external_memory": 2981,
 "peak_malloced_memory": 589280,

 "spaces": [
   {
     "name": "read_only_space",
     "size": 262144,
     "used_size": 32568,
     "available_size": 229256,
     "physical_size": 32888
   },
   {
     "name": "new_space",
     "size": 2097152,
     "used_size": 903392,
     "available_size": 143904,
     "physical_size": 1856136
   },
   ...
 ]
}
```

Bug: v8:9186
Change-Id: I0d07aa37b65d45778d6b47dbe6e07a9dd25d1097
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1619763Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#61667}